Anti-immunoglobulin G in the haemagglutination inhibition test for Newcastle disease virus antibody assays.

The effect of incorporation of species-specific anti-immunoglobulin G (anti-IgG) antisera into the haemagglutination-inhibition (HI) test for antibodies to Newcastle disease viruses (NDV) was investigated. Experiments were made on the specificity of the rise in titre and on the reproducibility of results, and routine examinations of sera were carried out. A special object was to scrutinize the specificity of low titres in conventional tests and to investigate the possibility of eliciting measurable titres from sera negative by conventional methods by adding anti-IgG. Despite the good accuracy and the acceptable reproducibility of the anti-immunoglobulin G haemagglutination-inhibition test, only 66% of sera positive in conventional haemagglutination-inhibition test yielded a rise of titres. Neither age nor 2-mercaptoethanol-sensitivity of haemagglutination-inhibiting antibodies seemed to be responsible for this property. A possible reason why not every serum that is positive in the normal test shows an increase in titre might be the physico-chemical properties of avian antiviral sera or immunoglobulins. The findings did not achieve the expectations mentioned earlier. If it were possible, however, to determine the reasons for the differences in the behaviour of NDV-antisera, the anti-IgG test could be of value in the understanding of the HI-test and for the further study of Newcastle disease serology.